1. Field of Invention
This invention relates generally to filter cartridges which contain membranes which are useful in separating materials. More particularly, this invention relates to processes and compositions which are used to end cap a filter cartridge.
2. Description of the Prior Art and More Particular Background
Polymeric membranes are well known. These membranes may generally be classified according to their retentivity, i.e., the sizes of particles which they retain, or according to their effective pore size, as either ultrafilter membranes, which have the finest pores, or microporous (or microfilter) membranes, which have coarser pores. The dividing line between ultrafilter membranes and microfilter membranes is between approximately 0.025 and 0.050 micrometers in pore size or smallest retained particle.
Membranes may also be classified according to the porosity difference or similarity of their two faces. Thus, membranes may be classified as symmetrical when the two faces have similar porosity, or as asymmetrical when the two faces differ in porosity.
An important characteristic of a membrane is its permeability to water which is measured by the volume of pure water which passes through a unit area of membrane per unit time. Water permeability is customarily expressed in units of cm/min-psi which represents the macroscopic velocity in cm/min at which water flows through the membrane when the driving pressure is one psi.
The flow of water through the membrane is, within wide limits, directly proportional to the applied pressure. In general, the permeability to water decreases as the retentivity of the membrane to solutes increases, because smaller pores offer more resistance to flow. This relationship, however, is not a simple one since the retentivity depends on the single smallest pore encountered by the liquid in passing through the membrane, whereas the resistance to flow depends on the cumulative effect of all the pores through which this liquid must pass. Hence, membranes of similar solute retention having uniform pores throughout their entire thickness have lower permeabilities than those whose retentivity is due to a thin skin having the same pore size combined with a body or substrate of much larger pores. In other words, symmetrical membranes offer more resistance to fluid flow and therefore have slower flow rates compared to asymmetrical membranes of similar retentivity.
In addition to their retention characteristics, membranes may be characterized by their ability to resist plugging or their dirt-holding capacity. Plugging refers to a reduction of the filtration rate during the filtering operation as a function of the amount of liquid passing the membrane. In order to extend the lifetime of a membrane in a given filter operation, it is customary to prefilter the fluid through a membrane or filter having higher flow rates and lesser retentivities, but still the ability to reduce severe fouling, or blocking, of the final membrane filter.
Structurally, membranes vary greatly and may generally be classified as either reticulated or granular. In the former, there is a three-dimensional open network of interconnecting fibrous strands and open interstitial flow channels. In the granular type structure, however, incompletely coalesced solid particles called granules leave an interconnected network of pores between them. Reticulated membrane structures generally have a higher porosity than granular membrane structures. (Porosity of membranes is defined as (1- the relative density). Porosity is also defined as the ratio of the weight of a given volume of membrane to that of the bulk polymer forming the membrane.)
Polymeric membranes are generally made by preparing a solution of the polymer in a suitable solvent, forming the solution into a thin sheet, a hollow tube or hollow fiber, and then precipitating the polymer under controlled conditions. Precipitation may be carried out by solvent evaporation or by contacting the polymer solution with a nonsolvent.
U.S. Pat. No. 3,615,024 discloses a method of forming porous polymeric membranes which are described as being highly asymmetric. The membranes produced according to that method are only slightly asymmetric, however, and have a permeability to water which is only slightly higher than that of symmetrical membranes of the same retentivity.
Membranes may also be classified as composite, supported or integral. Composite membranes comprise a very thin retentive layer attached to a preformed porous support. In a supported membrane, the actual membrane is attached to a strong sheet material of negligible retentivity. Integral type membranes are formed in one and the same operation having layers of the same composition. These layers may have very different properties, depending, in general, on whether the membrane is symmetrical or asymmetrical.
The membrane is supported by a support layer and both are wrapped around a central core of the cartridge such that a fluid which contains particles passes through the core, support, and membrane. The membrane allows the fluid to pass through, but retains the particles. An outer cage holds the membrane and the support to the central core.
At the end of the filter cartridge is a cap--a so-called "end cap" which is used to seal the cartridge against leaks. The filter cartridge may be "end capped" by applying a potting compound or adhesive to the end cap and immersing the cartridge elements (central core, membrane, support, and outer cage) into the adhesive. When the adhesive dries, the cartridge is theoretically sealed.
Problems develop in practice, however. If a low viscosity resin were to be used as the potting compound or adhesive, the resin tends to wick or rise up through the capillaries of the membrane and support and create voids under the pack and loss of initial integrity of the cartridge or loss of integrity after the cartridge is autoclaved (as it must be for certain applications). Furthermore, if the potting compound or adhesive has too high a viscosity, there results insufficient wicking and sharp interfacial transitions which cause both initial and post-autoclave failures.
The problems of excessive wicking and the resulting voids under the pack is particularly severe in the case of asymmetric membranes which contain a comparatively high amount of large pores in the side of the membrane opposite the skin. The low viscosity resin is more susceptible to wicking through these large pores than it would be through a relatively more dense symmetric membrane.
The search has continued for improved processes for end capping filter cartridges and for improved potting compositions. This invention was made as a result of that search.